Rooted in bedrock, reaching for the sky

The tower will soar above the skyline, but right now there is only an open space by the Chicago River, a home to construction equipment and a matter of fascination to neighbors.

Construction of the Trump International Hotel & Tower Chicago will offer a three-year spectacle of modern construction as the building climbs, level by level, above its neighbors and into the ranks of the world's tallest buildings. When done, it will stand as tall as the John Hancock Center even before its controversial spire is added.

In full view of onlookers, the former Chicago Sun-Times building disappeared from its longtime spot in a matter of months.

Now, passersby watch as drills the size of telephone booths spin into muddy ground where the newspaper offices once stood, spewing damp earth into piles that steam briefly in the chilly air.

Unlike Chicago's other three giants--the Sears Tower, the Aon Center and the Hancock building--the Trump tower will be supported not by a framework of steel but by a spine and outriggers of concrete.

Without high-quality concrete, the structure would never support the building's 360,000-ton bulk--the weight of four aircraft carriers.

Without new chemical processes that make the wet cement more fluid or new pumping techniques to move it, it could never be pumped 92 stories and 1,125 feet into the air.

Without concrete, the building could never climb so high and still stay so thin.

The footprint of Trump's building will be 348 feet by 135 feet--not much bigger than the squat Sun-Times building.

"On a steel building, it would have had to be much wider," said William Baker, a structural engineer at Skidmore, Owings & Merrill, the architectural firm that designed both buildings.

"We wouldn't have been able to put a steel building on this site."

When the building is finished, a skin of stainless steel and glass will reflect the sun. The tower's shape will mirror the buildings around it, its faceted setbacks nodding to the 1920 Wrigley Building and monolithic 1973 IBM Building next door.

The building's designers are using the tower's concrete-swollen weight to defy heavy breezes. By making it too heavy to tip easily, the designers have pitted one of the oldest enemies of skyscrapers, gravity, against its other foe, the wind.

In addition, the setbacks and rounded edges will prevent vortices from organizing into mini-tornadoes, reducing the wind's power.

To further secure it, the building will be cantilevered into a section of the Earth's crust, a limestone formation 420 million years old and 110 feet underground, so the building will touch sky and bedrock at the same time.

Though it will outreach its neighbors, the Trump tower must start far beneath them, on pillars extending like stilts into the ground. The holes for those 4-foot-wide pillars are being drilled now. Under the building, every 30 feet around its perimeter, steel-reinforced cement will be poured.

On top of that, an 8,400-ton concrete pad the size of a river barge will be built. From that pad the building's spine will rise, climbing as Wabash Avenue is rebuilt between the IBM Building and the future Trump tower.

The spine will consist of five gigantic concrete walls, each shaped like an I-beam standing on its 45-foot-wide end. At about eight stories, the exterior columns will begin to follow, with a concrete slab between the columns at each story for a floor.

It will climb like this: Spine, columns, slab. Spine, columns, slab. Eight to 10 floors below the highest slab, the curtain wall will rise on the outside of the structure. As the building climbs, it will narrow, the spine dwindling to two parallel I-beams from five.

As the building takes root, it has become an attraction for the curious. The two-story-deep construction pit extends toward the IBM Building, and a nearby sidewalk has become an observation gallery.

Crane platforms with their coils of heavy cable are at eye level. Viewers see the tops of bulldozers.

One day, the workers will point to the tall shape in the city skyline with a sense of ownership.

Earthmovers heap dirt into piles. The heads of hydraulic dinosaurs bob into the rubble, crushing and pulling scrap metal in their jaws. Rolled steel dangling a dozen feet above the ground is lowered into rows. Sounds echo from the pit.

Above, people watch.

"It looks weird," said bicycle messenger Lee Towne, 47, of Chicago.

"It looks like a whole different site."

- - -

Designed to fit

When built in downtown Chicago, the Trump International Hotel & Tower Chicago will be one of the tallest concrete buildings in the world.

Because the Trump tower needed to fit within the site of the former Chicago Sun-Times, architects chose to build with concrete instead of steel. Concrete allows them to build the building taller without making it wider. Using fluid cement and new construction techniques, workers can pump cement hundreds of feet up into the air.

ADVANTAGES OF CONCRETE

- Does not require a large base for construction

- Virtually fireproof, easier to isolate a high-rise fire

- Floor slabs typically can be thinner, allowing more living units to be built

Designed to handle wind

The asymmetrical design and weight from concrete give the 360,000-ton tower additional stability against the wind

DISRUPTING FORMATION OF VORTICES

Vortices are small forces of whirling wind that can cause the tower to sway.

- Curved edges allow the approaching wind to wrap around the building's corners more easily than around a building without an aerodynamic design.

- Setbacks further disrupt the wind's constant push on the building's shaft and limit formation of vortices.

Building from the ground up

The building process begins several stories below ground. The tower will be layered floor by floor until it reaches 92 stories.

Core: Spine of the building

Columns and slabs: Provide support for each new floor built above.

Caissons: Reach about 110 feet into the bedrock and act as stilts to support the structure.

1. Laying the foundation

Caisson shafts are drilled into the ground, then filled with cement.

2. Forming the concrete structure

The core, columns and slabs are added. This process is repeated for each floor.

3. Enclosing the building

Stainless steel, glass and aluminum panels are attached at each floor.

4. Finishing the interior

Interior components, such as drywall, electrical and mechanical systems, elevators and doors, are added.

CONSTRUCTION TIMELINE

Approximate times

1. (Early 2006)

2. (Mid-2008)

3. (Mid-2008)

4. (Early 2009)

To build the tower with steel, the building would need to be 25 feet wider.

Concrete: 135 feet

Steel: 160 feet

Height excluding spire: 1,125 feet

Spire

Will rise to 1,360 feet, which will make the tower 90 feet shorter than the Sears Tower.